The successful use of extruded tubes of expanded polytetrafluoroethylene (ePTFE) as synthetic implantable vascular prostheses or tubular grafts is well known and documented. ePTFE, which has been validated through numerous clinical studies, is particularly suited for this purpose as it exhibits superior bio-compatibility and can be mechanically manipulated to form a well-defined porous microstructure known to promote endothelialization. Further, ePTFE has been proven to exhibit a low thrombogenic response in vascular applications. When seeded or infused with an active agent, the microporous structure of ePTFE, comprising nodes and fibrils, controls natural tissue ingrowth and cell endothelialization when implanted in the vascular system. This ability contributes to patency of the tubular graft and long term healing.
In U.S. Pat. No. 6,436,135 to Goldfarb, the microstructure of a synthetic vascular prostheses formed of ePTFE is defined by irregularly spaced nodes interconnected by elongated fibrils. The methods by which these types of structures are produced have been known for more than three decades. In such a structure, the distance between the node surfaces spanned by the fibrils is defined as the inter-nodal distance (IND).
An ePTFE-based vascular prosthesis having a specific IND range can be developed with a given porosity and/or pore size range to enhance tissue ingrowth and cell endothelialization along the inner and outer surface of the prosthesis. The IND range is generally small enough to prevent transmural blood flow and thrombosis but is generally not less than the maximum dimension of the average red blood cell (e.g., between about 6 and 8 μm). Vascular prostheses based on ePTFE are thus inherently porous. The porosity of an ePTFE vascular prosthesis is controlled by the mechanical formation of the IND and/or the microporous structure of the tube.
One exemplary vascular prosthesis is a stent, which is a medical device commonly used to restore and maintain body passages, such as blood vessels. Often, biocompatible materials can be provided on the inner and/or outer surfaces of the stent to reduce reactions associated with contact between the stent and the body. However, it is difficult with conventional devices to manipulate certain properties such as mechanical properties, cellular proliferation, cellular permeability, fluid permeability, adhesion to a structural frame, and/or incorporation of one or more active therapeutic components. Although coverings can sometimes be used to alter the properties of devices, stents generally have complex geometries that cannot be readily covered with covering materials such as ePTFE. Thus, a need exists for materials and processes that address these concerns. Specifically, it would be beneficial to provide methods of providing implantable prosthetic devices, including stents, with properties that can be tailored for various purposes.